HIGH FDG UPTAKE ON PRE-RADIOTHERAPY PET/CT AND PREFERENTIAL SITES OF LOCAL RELAPSE AFTER CHEMORADIOTHERAPY FOR LOCALLY ADVANCED HEAD AND NECK CANCER

2707a Objectives The potential benefits of 18F-fluorodesoxyglucose positron emission tomography (FDG-PET/CT) imaging for radiotherapy (RT) planning treatment of head and neck cancer (HNC) are increasingly being recognized. It has been suggested that intra-tumor sub-volumes with high FDG avidity are potential target for selected dose escalation. Our aims were to demonstrate that pre-RT FDG PET/CT can identify intra-tumor sites at increased risk of local relapse (LR) after RT and to determine an optimal maximum standardized uptake value (SUVmax) threshold to delineate smaller RT target volumes that would facilitate RT dose escalation without impaired tolerance. Methods The study included 72 consecutive patients with locally advanced HNC treated by radiotherapy ± concurrent chemotherapy. All patients underwent FDG PET/CT at initial staging (PETA ) and during systematic follow-up (PETR ) in a single institution. FDG PET/CT acquisitions were co-registered on the initial CT scan with a rigid method. Various subvolumes (AX; X=30, 40, 50, 60, 70, 80 and 90% SUVmax thresholds) within the initial tumor and in the subsequent LR (RX; X=40 and 70% SUVmax thresholds) were pasted on the initial PET/CT and compared together [Dice, Jaccard, overlap fraction (OF), common volume/baseline volume, common volume/recurrent volume]. Results : Nineteen patients (26%) had LR. Using a 40% of SUVmax threshold, initial metabolic tumor volume (MTV) was significantly higher in all relapse patients (local and distant relapse) than in controlled patients (mean: 11.3 ± 9.8 vs. 5.1 ± 4.9 cc, p = 0.001) as well as total lesion glycolysis (TLG) (mean: 134.6 ± 116 vs. 60.6 ± 80.4, p=0.002). For both using methods, the overlap index between A30,, A40, and A50 sub-volumes on PETA and the whole metabolic volume of recurrence R40 and R70, on PETR showed a moderate agreement (between 0.40 and 0.60). Conclusions We confirm that TLG and MTV are independently correlated with recurrence-free survival in patients with HNC. Due to sub-optimal co-registration, we could not reach high overlap index values between initial tumor and recurrence sub-volumes. Further larger prospective studies with FDG-PET/CT performed in the same RT position and with a validated elastic registration method are needed.